1. Field of the Invention
This invention relates to circuit boards and, more particularly, to a method of forming a conductive pattern on a circuit board.
2. Background Art
It is well known to make conductive patterns on circuit boards for electronic equipment utilizing a ceramic circuit board with a metal layer formed in a predetermined pattern defining a conductive path on the board. Typically, the circuit board may be made from sintered aluminum oxide, sintered aluminum nitride, sintered silicon carbide, or the like, or glass board with good adhesion properties.
In a conventional electroless plating method, the surface of the ceramic board is roughened by a chemical or physical process. The board is then treated in a concentrated hydrochloric acid solution of SnCl2 to sensitize the surface thereof. The board is treated in a concentrated hydrochloric acid solution of PdCl2 to form plating metal nuclei of Pd on the surface. The board is then placed in a plating bath containing gold, silver, copper, nickel, or the like to form a metal layer thereon. This plating process is performed without the use of an electric current, i.e. in an electroless process.
While this plating method is desirable from the standpoint of cost, it has some drawbacks. The surface of the board is roughened to improve adhesive strength and to increase the surface area thereof so that, for example, palladium particles are bonded as catalytic nuclei for plating film deposition on the board surface. However, the adhesion between the plating particles and the board surface may not be as strong as is desirable. Further, the high frequency characteristics of the board may deteriorate by reason of the board surface being roughened. Further, this plating process cannot be used for a board whose surface cannot be roughened. Among these boards are high purity alumina and aluminum nitride boards.
An alternative conventional method of forming circuit boards involves vapor deposition of a metal. In the process, a metal is evaporated by, for example, argon sputtering to deposit the metal on a ceramic board, with the board placed in a vacuum. Since generally an adequate adhesive force between the board and gold, silver, copper, nickel or the like cannot be realized, an intermediate layer having better adhesive characteristics is commonly formed against the board. A metal film is then formed to produce the desired conductive path.
The vapor deposition process is desirable in that the conductive path can be formed on high purity alumina, aluminum nitride, or the like on which conductive paths cannot effectively be formed by the above-described electroless plating method. The metal film can be formed on the board without surface roughening, as a consequence of which high frequency characteristics are not compromised. However, the vapor deposition method is more costly than the electroless plating process described above.
In one form, the invention is directed to a method of forming a conductive circuit pattern on a circuit board having a first region, on which a desired conductive circuit pattern is to be formed, and a second region. The method includes the step of applying a coating including a solution with conductive particles to the circuit board. The coating is heated to adhere the conductive particles to the circuit board. The conductive particles are removed in the second region. The second region is shielded and, with the second region shielded, a conductive film is formed on the first region.
In one form, the circuit board is made from a ceramic material.
The circuit board may be made from at least one of a) sintered aluminum oxide, b) sintered aluminum nitride, and c) sintered barium titanate.
In one form, the coating is a fine particle dispersed solution with fine particles that are at least one of copper and copper oxide.
In one form, the particles have a diameter of 1-500 nm.
In one form, the step of heating the coating involves burning the coating in an inert gas atmosphere to adhere copper fine particles as plating catalytic nuclei to the circuit board.
The heating step may involve burning at a temperature of 500-1100xc2x0 C. for one to sixty minutes.
In one form, the step of removing the conductive particles may involve removing the plating catalytic nuclei by etching.
In one form, the step of shielding the second region may involve applying a resist film.
The resist film may be one of a) a dry film and b) resist ink.
The step of forming a conductive film may involve forming a copper film by electroless copper plating.
The method may further include the step of removing the resist film after forming the copper film.
In one form, the fine particle dispersed solution consists of fine particles dispersed in a solvent.
The fine particles may be copper that is present in an amount of 0.01-80% by weight.
The solvent may be, for example, at least one of a) xcex1-terpineol, b) methanol, c) ethanol, d) water, e) carbitol, and f) methacresol.
In one form, the fine particles are dispersed in the solvent in an amount of 0.001-10 weight %.
The coating may be applied to a surface of the circuit board to a thickness of 10-1000 nm.
In one form, the step of drying the coating involves heating at a temperature between 80xc2x0 C. and 500xc2x0 C.
The method may further include the steps of applying a negative film having a transparent portion corresponding to the first region, and curing the resist film.
In one form, the method further includes the steps of removing the negative film and removing the resist film in the second region by developing.
The method may further include the step of removing the resist film in the first region. In one form, this is carried out using a solvent.
In one form, the coating consists of fine particles dispersed in a polymer. Alternatively, the fine particles may be dispersed in an oligomer matrix.
In one form, the coating consists of at least one of a polymer, an oligomer, and a solvent applied to a circuit board as a film, and the fine particles include at least one of copper and copper oxide applied to the film.
The film may be mixed with a solvent to effect dispersal of the particles.
In another form, the invention is directed to a method of forming a conductive circuit pattern on a circuit board having a first region on which a desired conductive circuit is to be formed, and a second region. The method involves the step of applying a coating consisting of a first solution including conductive particles of a diameter of 1-500 nm to an unroughened surface of the circuit board. The coating is heated to bond the conductive particles to the circuit board. The conductive particles are removed in the second region.
In one form, the conductive particles are at least one of copper and copper oxide.
The method may further include the steps of shielding the second region and, with the second region shielded, forming a conductive film in the first region.